Crack Formation during Foam Pattern Firing in the Investment Casting Process

The application of rigid polymeric foam for large investment casting patterns with complex geometries can improve the dimensional tolerances and the surface quality of the casting. However, these pattern materials have a tendency to promote crack formation in investment casting shells during pattern removal by firing. Experimental methods were combined with finite element modeling to predict stress in the shell. The model takes into consideration the thermal and mechanical properties of the pattern and the shell materials to determine the heat transfer and thermal expansion stresses developed in the shell during firing. The thermal and mechanical properties of the pattern and shell were obtained from experimental tests. A 3D nonlinear finite element model was developed to predict possible crack formation in the shells during pattern removal. The effects of the thermo-mechanical properties of the foam and the shell, as well as the firing process parameters were modeled, and extreme cases were experimentally validated. Recommendations for firing process parameters and pattern design to decrease stress and eliminate crack formation in the shell were formulated. Keywords: ceramic shell, investment casting, crack, stress modeling, molding, foam patter